Rotary ultrasonic motors using piezoelectric materials were studied by H. V Barth of the United States in 1973, V. V Lavrinenco of the Soviet Union in 1973, Sashida of Japan, etc.
A representative rotary ultrasonic motor is a single-phase rotary motor developed by Physik Instrumente (PI).
FIG. 1 is a schematic view of a rotary ultrasonic motor in the art.
Referring to FIG. 1, a rotor 30 is placed above a cylindrical piezoelectric actuator 10 placed at the center of the motor. A semispherical pusher 20 is placed between the rotor 30 and the piezoelectric actuator 10 to minimize friction while allowing smooth rotation.
Further, the rotary ultrasonic motor includes a shaft 40 for transferring motion of the rotor 30 outside, a base 50 for supporting the shaft 40, a spider spring 60, and a coupling ring 70.
All of these components may be protected by a cylindrical case 80.
Here, in order to generate vibration on a tangential axis with the rotor 30, the piezoelectric actuator 10 is divided into a plurality of electrodes (free electrodes and active electrodes) along an outer surface of the piezoelectric actuator 10. The detailed shape of the piezoelectric actuator 10 and vibration simulation thereof are as follows.
FIGS. 2 and 3 are schematic views showing an operational principle of the ultrasonic motor based on computer simulation.
Referring to FIGS. 2 and 3, the cylindrical piezoelectric actuator 10 has electrodes which are divided at constant intervals, and the piezoelectric actuator 10 expands in a thickness direction at electrode sections (active electrode sections) to which voltage is applied and maintains its original shape at electrode sections (free electrode sections) adjacent to the active electrode sections.
Thus, as shown, a simulated shape 90 of the piezoelectric actuator having a waveform is obtained. As can be seen from the simulated shape, the rotor is rotated in a direction in which the semispherical pushers 20 are inclined.
However, it is difficult to commercialize this type of ultrasonic motor due to its low efficiency.
Thus, T. Sashida developed an ultrasonic motor using traveling waves in 1982.
FIGS. 4 and 5 are schematic views of an ultrasonic motor using traveling waves in the art.
FIGS. 4 and 5 show a principle of rotating a rotor 35 by traveling waves formed from two standing waves in an annular piezoelectric actuator 15.
In addition, the ultrasonic motor includes common components such as a shaft 45, base 55, bearing 65, and case 85, and detailed descriptions thereof will be omitted.
As described above, unlike an electromagnetic motor generating drive power by interaction between electric current and a magnetic field, the ultrasonic motors in the art converts friction between a stator (piezoelectric actuator) vibrating due to ultrasonic waves and a mover into rotational force.
It is known that energy density of ultrasonic vibration energy theoretically reaches several hundreds of W/cm2, which is 5 to 10 times higher than conventional electric magnetic motors, and the ultrasonic motor has an advantage of generating high torque at low speed without generating EMI.
However, since the ultrasonic motor has a complex structure and requires high precision components, manufacture of the ultrasonic motor is troublesome, and it is difficult to achieve mass production and miniaturization of the ultrasonic motors.